1. Field of the Invention
The present invention relates to signal generating circuits and, more specifically, to a signal generating circuit that generates two signals differed in phase by 90 degrees from each other (hereinafter referred to as a reference phase signal and a quadrature signal, respectively).
2. Description of the Background Art
Wireless communications devices in recent years often incorporate a quadrature modulator, a quadrature demodulator, or an image rejection mixer. These components require the above-mentioned reference phase signal and quadrature signal.
FIG. 12 is a block diagram illustrating the circuit structure of a general quadrature modulator. In FIG. 12, the quadrature modulator includes a local oscillator 101, a 90-degree divider 102, two input terminals 103 and 104, two mixers 105 and 106, a combiner 107, and an output terminal 108.
The local oscillator 101 generates a local oscillation signal SLO for output to the 90-degree divider 102. Here, for the convenience of descriptions of embodiments, an oscillation frequency fc of the local oscillation signal 101 is taken as fc1. The 90-degree divider 102 generates a reference phase signal SREF and a quadrature signal SQR from the received local oscillation signal SLO. Here, the reference phase signal SREF and the quadrature signal SQR ideally are different in phase by 90 degrees and are equal in amplitude to each other. This reference phase signal SRF is supplied to the mixer 105, and this the quadrature signal SRA is supplied to the mixer 106.
Three exemplary implementations of the 90-degree divider 102 are described below with reference to FIGS. 13 through 15. FIG. 13 is a schematic diagram illustrating the circuit configuration of an RC shifter, which is a first exemplary implementation of the 90-degree divider 102. In FIG. 13, the RC shifter includes an input terminal 201, a divider 202, a serial capacitor 203, a parallel resistor 204, a serial resistor 205, a parallel capacitor 207, and two output terminals 207 and 208. In the above-structured RC shifter, when the above-mentioned local oscillation signal SLO is supplied to the input terminal 201, the above-mentioned reference phase signal SREF and quadrature signal SQR are output from the output terminals 207 and 208, respectively. A 90-degree divider implemented by the above-structured RC shifter is disclosed in, for example, “RF Microelectronics” authored by Behzad Razavi, p138.
FIG. 14 is a schematic diagram illustrating the circuit configuration of a polyphase filter, which is a second implementation of the 90-degree divider 102. In FIG. 14, the polyphase filter includes two input terminals 211 and 212, four resistors 213 through 216, four capacitors 217, 218, 219, and 2110, and four output terminals 2111 through 2114. In the above-structured polyphase filter, the input terminals 211 and 212 are supplied with a set of differential signals. This set of differential signals is generated from the above-mentioned local oscillation signal SLO. One differential signal is supplied to the input terminal 211 and, in general, is equal in phase to the local oscillation signal SLO. The other differential signal is supplied to the input terminal 212 and, in general, has a reverse phase of the one differential signal. With such differential signals, the above-mentioned reference phase signal SRF and quadrature signal SQF are output from the output terminals 2111 and 2113, respectively. These two signals SREF and SQR are also output from the output terminals 2112 and 2114, respectively. As such, the above-structured polyphase filter includes the input terminals 211 and 212 that receive differential signals, and therefore is suitable for a semiconductor integrated circuit often using a differential circuit.
Furthermore, two or more polyphase filters of FIG. 14 can be connected in series as illustrated in FIG. 15 to widen the band of the 90-degree divider 102.
Referring back to FIG. 12, the mixers 105 and 106 are further supplied with a baseband signal SBB through the input terminals 103 and 104. The mixer 105 mixes the input reference phase signal SREF and the input baseband signal SBB to generate a reference phase modulated signal MSREF for output to the combiner 107. The mixer 106 mixes the input quadrature signal SQR and the input baseband signal SBB to generate a quadrature modulated signal MSQR that is orthogonal in phase to the reference phase modulate signal MSREF. This quadrature modulated signal MSQR is also output to the combiner 107.
The combiner 107 combines the input reference phase modulated signal MSREF and the quadrature modulated signal MSQR to generate a composite signal SMP. With this combining, image components are rejected in the composite signal SMP. Here, a ratio of rejection of image components depends to a large degree on a difference in amplitude and/or phase occurring in the 90-degree divider 102. Therefore, the 90-degree divider 102 incorporated in a quadrature modulator has to be able to generate highly-accurate reference phase signal SRF and quadrature signal SQR that are equal in amplitude and highly orthogonal to each other.
Note that a quadrature demodulator performs operations in reverse to those of the quadrature modulator, and is therefore not described or illustrated herein.
FIG. 16 is a schematic diagram illustrating the circuit configuration of a general image rejection mixer. In FIG. 16, the image rejection mixer is different from the quadrature modulator of FIG. 12 only in that the two input terminals 103 and 104 are replaced by a single input terminal 111 and a 90-degree divider 112. Therefore, in FIG. 16, components equivalent in structure to those in FIG. 12 are provided with the same reference numbers.
The input terminal 111 is given an intermediate frequency (IF) signal SIF, and is then input to the 90-degree divider 112. As with the 90-degree divider 102, the 90-degree divider 112 generates a reference phase intermediate frequency (IF) signal IFSREF and a quadrature intermediate frequency (IF) signal IFSQR from the input IF signal SIF. Here, the reference phase IF signal IFSREF, and the quadrature IF signal IFSQR are different in phase by 90 degrees and equal in amplitude to each other. This reference phase IF signal IFSREF is supplied to the mixer 105, while the quadrature IF signal IFSQR is output to the mixer 106.
The mixer 105 mixes the input reference phase IF signal IFSREF and the input reference phase signal SREF to generate a reference phase modulated signal MSREF for output to the combiner 107. The mixer 106 mixes the input quadrature IF signal IFSQR and the input quadrature signal SQR to generate a quadrature modulated signal MSQR for output to the combiner 107. The combiner 107 combines the input reference phase modulated signal MSREF and the input quadrature modulated signal MSQR to generate a composite signal SMP.
However, the above-structured 90-degree divider 102 has a problem such that, accurate reference phase signal SREF and quadrature signal SQR cannot be generated as the local oscillation signal SLO becomes higher in frequency. More specifically, for the purpose of making the 90-degree divider 102 structured as illustrated in any of FIGS. 13 through 15 generate ideal reference phase signal SREF and quadrature signal SQR, a capacitance C, a resistance R, and the oscillation frequency fc of the local oscillator 101 should be selected so as to satisfy the following equation (1):fc=½πRC  (1)
As evident from the above equation (1), as fc becomes higher, R and/or C becomes lower, making the circuit elements more susceptible to their structural variations and/or thermal deviations. As a result, a difference in amplitude and/or phase occurs between the reference phase signal SREF and the quadrature signal SQR.
Especially, in a semiconductor integrated circuit, when a difference between a contact resistance between wiring layers and a resistance on the circuit becomes smaller, variations of the resistance becomes larger. Also, when the capacitance on the circuit becomes smaller, stray capacitance on a circuit board or wiring becomes non negligible, thereby making it more difficult to achieve a highly-accurate 90-degree divider.